1. Field of the Invention
This invention relates to compositions and methods for treating subterranean formations. More specifically, the present invention relates to compositions and methods for plugging thief zones and fractures in subterranean formations.
2. Description of the Related Art
Water flooding is an important method that can be used to recover oil from both sandstone and carbonate reservoirs. Only approximately one third of the original oil in place (OOIP) is recovered by primary and secondary recovery processes, typically leaving two-thirds of the OOIP trapped in reservoirs as residual oil after water flooding. Approximately 50% of discovered oil in the world is in carbonate reservoirs with most of these reservoirs having natural fractures.
Most of carbonated reservoirs in areas of the world, such as Saudi Arabia, are completely underlain by water with a large oil column and the gas cap. The gas cap plays a major role in production of oil fields. To effectively develop carbonated reservoirs having a large gas cap, early gas breakthrough and gas slippage need to be prevented.
A completion methodology referred to as water flooding has been used to improve oil recovery and minimize water intrusion in the formation. Water flooding takes into consideration the placement of a fracture barrier at the toe of the producing well to delay water intrusion and improve oil recovery efficiency. Others have developed in-depth water flood conformance improvement tools that use time-delayed, highly expandable particulate material (for example, Kernel particles/Bright water) that can improve the sweep efficiency of a water flood. The expanded Kernel particles can provide resistance to fluid flow in the formation. The material appears to plug pores at up to about 125 feet from the injector, which generally can result in a reduction of water cut by more than 60%. The Kernel particles treatment, however, was intended for diverting water in the matrix, not the fractures.
High residual oil saturation is in part cause by poor sweep in fractured reservoirs. Conventional water flooding is used to displace oil from the permeable zones or fractures, bypassing substantial amounts of trapped oil in the lower permeability zones. If the carbonate reservoir is preferentially oil-wet, the matrix will retain oil and high residual oil saturation in the matrix when injected water breakthroughs from fractures and/or high permeability zones (“thief zones”).
The thief zones or fractures are often a rather long distant away from the wellbore. It is preferable to leave the vicinity of the wellbore open and permeable and to plug only the area considered to be the thief zones. Therefore, plugging the right location is a delicate undertaking that requires precise distance, timing and solution properties.
A need exists for compositions and methods that can be used to plug pores/fractures in the thief zone. There is also a need to plug thief zones or fractures in the formations that are farther away (for example, greater than 125 feet) from the injector or wellbore than other prior methods have been able to achieve. It would be advantageous of such compositions and methods could be used to control the locations that are to be plugged.